(1) Field of the Invention
The present invention relates to a filter material suitable for filtering a fluid containing an acidic substance or an alkaline substance over a long period of time.
(2) Description of the Related Art
As the filter material for a fluid containing an acidic substance or alkaline substance such as sulfur dioxide (SO.sub.2), sulfur trioxide (SO.sub.3), sulfuric acid (H.sub.2 SO.sub.4), hydrochloric acid (HCl), ammonia (NH.sub.3) or sodium hydroxide (NaOH), an inorganic fiber such as a glass fiber or asbestos fiber or a fibrous structure composed of an organic heat-resistant fiber such as an aromatic polyamide fiber, the fiber surface of which is covered with a fluoropolymer resin, a melamine resin or a mixture thereof has been mainly used at high temperatures (higher than 150.degree. C.), and a fibrous structure composed of a chemical-resistant fiber such as an acrylic fiber has been mainly used at a low temperature (lower than 150.degree. C.). However, when a fibrous structure of an inorganic fiber excellent in the acid resistance and alkali resistance (hereinafter referred to as "chemical resistance") is used as the filter material, since the flexural property is poor, the filter material is seriously damaged when dusts are shaked down or brushed off, and hence, the life is short. Moreover, it has been recently pointed out that a bad influence is given to a human body when a worker is engaged with the production over a long period of time, and this becomes a great social problem.
Fibrous structures comprising an organic heat-resistant fiber such as an aromatic polyamide fiber, the surface of which is covered with a fluoropolymer resin, a melamine resin or a mixture thereof, are disclosed, for example, in Japanese Examined Patent Publication No. 51-34586 and Japanese Unexamined Patent Publication No. 56-124414. A fibrous structure composed of a chemical-resistant fiber material such as an acrylic fiber is satisfactory in the flexural durability and handling property and has a considerably good chemical resistance. For example, a filter material of this type has been proved to be applicable to filtering of dusts in an acidic gas atmosphere having an SO.sub.x concentration of 50 to 100 ppm and has been practically used for this purpose. From the resource-saving viewpoint, conversion of low-sulfur-content heavy oil to high-sulfur-content heavy oil or coal as the fuel has been advanced at a high pitch, and it is required to filter an acidic gas having an SO.sub.x concentration higher than several hundred ppm in a waste gas at a temperature higher than 150.degree. C. Accordingly, development of a filter material having a high chemical resistance and a high heat resistance in combination is eagerly desired. An organic chemical-resistant fibrous structure heretofore used is insufficient and for the above-mentioned purpose a fibrous structure composed of a glass fiber is used.